Conventionally, the following techniques have been proposed, and have greatly developed in response to widespread use of a liquid crystal display: (i) an integrated circuit device technique in which a single crystal silicon substrate is processed so that several hundred millions of transistors are formed on the single crystal silicon substrate and (ii) a thin-film transistor (TFT) technique in which a polycrystalline semiconductor film such as a polycrystalline silicon film is formed on a light-transmitting amorphous material such as a glass substrate, and is then processed so as to be converted into transistors from which a picture element, a switching element, a driver and the like of a liquid crystal display device are made.
(Integrated Circuit Element Technique)
Of the above-mentioned techniques, for example, the integrated circuit element technique is such that a commercially available single crystal silicon wafer having a thickness of less than 1 mm and a diameter of approximately 200 mm is processed so that a large number of transistors are formed on the silicon wafer.
(Thin-Film Transistor Technique)
Meanwhile, in a case where the thin-film transistor technique is applied to a TFT-liquid crystal display device, an amorphous silicon film, formed on a light-transmitting (amorphous high distortion point) non-alkali glass substrate, is melted and polycrystalized by heating such as laser heating, and is then processed so as to be converted into a MOS transistor which functions as a switching element of the TFT-liquid crystal display device, for example.
(Semiconductor Apparatus)
Further, a technique in which a silicon film, especially a single crystal silicon film is formed on an insulator by a transfer method is proposed. A semiconductor apparatus manufactured by the transfer method may be referred to as a SOI (Silicon On Insulator) substrate.
(Semiconductor Apparatus in Integrated Circuit)
In the field of integrated circuit, the semiconductor apparatus is used to improve a function of a semiconductor element such as a transistor.
Specifically, in a case where a transistor is manufactured with the use of a semiconductor apparatus, elements are completely separated. This imposes little restriction on operation, thereby allowing the transistor to have good property and high performance.
A substrate used in the field of integrated circuit is not particularly limited as long as it is an insulator (or an insulating film). The substrate may be transparent or non-transparent, and may be crystalline or amorphous.
(Display Device)
In contrast, in the field of display device such as a TFT-LCD (Liquid Crystal Display) device and a TFT-OLED (Organic Light Emitting Diode) display device, such a substrate needs to be transparent, and typically is an amorphous substrate such as a glass substrate.
An amorphous silicon film or a polysilicon film is formed on the substrate, and the TFT is formed based on this. The TFT is used as a switching element for so-called active matrix driving of the display device.
Further, in order to integrate, on the substrate, devices used for the active matrix driving such as a peripheral driver and a timing controller, a silicon film formation substrate that is higher in performance has been studied.
(Polysilicon Film)
Conventionally, in a case where a polysilicon film is used as a silicon film, a localized state in a band gap that is caused by incomplete crystal or a localized state in a defective band gap in the vicinity of a crystal grain boundary easily occurs. The occurrence of such a localized state causes a reduction in mobility and an increase in subthreshold coefficient (S coefficient), thereby deteriorating performance of a transistor.
Further, in a case where a silicon film in a polysilicon film has incomplete crystal, a fixed charge is easily formed on an interface between the silicon film and a gate insulating film in the thin-film transistor. The formation of such a fixed charge makes it difficult (i) to control a threshold voltage of the thin-film transistor and (ii) to attain a desired threshold voltage value.
Especially in a case where a polysilicon film is formed on a large glass substrate, it is difficult to miniaturize a device such as a transistor. As a result, it is difficult to improve performance and to increase speed of such a device.
Further, in a case where a polysilicon film is obtained by heating an amorphous silicon film by irradiation with a laser beam, there is a large variation in mobility and threshold voltage of a transistor. This is because fluctuation in irradiation energy of the laser beam causes an unevenness in grain size of the polysilicon film thus obtained.
Further, in a case where a polysilicon film is obtained by heating an amorphous silicon film by irradiation with a laser beam, temperature of the silicon film temporarily rises nearly to a melting point of silicon due to heating by the laser beam. This causes an alkali metal and the like contained in a glass substrate to be diffused into the silicon film, thereby lowering properties of the transistor.
(Single Crystal)
In order to solve the problems caused by use of a polysilicon film, a device using single crystal silicon has been studied.
For example, Patent Literature 1 discloses such a device using single crystal silicon.
Specifically, Patent Literature 1 discloses a semiconductor apparatus in which a single crystal silicon film is provided on a coating film formed on an insulating substrate (glass substrate) for semiconductor apparatus. In the semiconductor apparatus disclosed in Patent Literature 1, hydrogen ions are implanted into a single crystal silicon substrate so that the single crystal silicon substrate is divided. Thus, the single crystal silicon film is obtained.